Synchronous rectification is a technology for improving efficiency by replacing a rectifying diode with a power metal oxide semiconductor field-effect transistor (MOSFET). MOSFET is a voltage controlled device which has a linear voltage-current characteristic when turned ON. So the gate voltage of the power MOSFET used as a synchronous rectifier needs to be in phase with the rectified voltage.
FIG. 1 illustrates waveforms of a smart synchronous rectification, wherein Vds indicates the drain-source voltage of a secondary switch (the synchronous rectifier), Isec indicates the current flowing through a secondary winding, CTRLS indicates the control signal of the secondary switch. The drain-source voltage Vds is compared with two threshold voltage, such as −70 mV and −500 mV. When a body diode of the secondary switch is turned on, the drain-source voltage Vds drops rapidly. The secondary switch is turned on when the drain-source voltage Vds drops below −500 mV. When a primary switch is turned on, the drain-source voltage Vds rises rapidly. The secondary switch is turned off when the drain-source voltage Vds rises up to −70 mV.
The smart synchronous rectification shown in FIG. 1 turns off the secondary switch after the primary switch is on. This may induce shoot through between the primary switch and the secondary switch. Furthermore, because of the characteristic of the secondary switch and delay of the control circuit, there may exist delay to turn off the secondary switch after the drain-source voltage Vds rises to reach −70 mV, which obviously worsen the case.